1. Field of the Invention
This invention relates generally to a pressure tank and, more particularly, to a hydrogen pressure tank that includes a specially designed connection assembly for providing a gas to or removing the gas from the tank, while maintaining a gas tight seal.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cell systems as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
Typically hydrogen is stored in a compressed gas tank under high pressure on the vehicle to provide the hydrogen necessary for the fuel cell system. The pressure in the compressed tank can be upwards of 700 bar. In one known design, the compressed tank includes an inner plastic liner that provides a gas tight seal for the hydrogen, and an outer carbon fiber composite layer that provides the structural integrity of the tank. At least one pressure regulator is typically provided that reduces the pressure of the hydrogen within the tank to a pressure suitable for the fuel cell system.
As the hydrogen is removed from the compressed tank, the pressure of the hydrogen in the tank will decrease. When the pressure of a gas is reduced and the volume does not change, the temperature of the gas will also decrease. The effect of the decrease in the temperature will be limited because heat is transferred from the environment into the tank. If the flow rate of the hydrogen flowing out of the tank is high enough and/or the temperature of the environment is low enough, the temperature in the tank can fall below −80° C. Typically it is possible to limit the hydrogen flow rate so that −80° C. is the lowest temperature that occurs within the tank. Further, when the tank is being filled with hydrogen, the temperature of the hydrogen can increase to 80° C. due to the compression of the hydrogen inside of the tank, providing a temperature swing of −80° C. to 80° C. The materials that can seal hydrogen in this temperature range are difficult to produce.
If the temperature of the hydrogen within the tank decreases beyond a certain temperature, such as −80° C. around the liner and −40° C. at the tank seals, including O-rings and other sealings, the materials become brittle and possibly damaged, affecting the tank's gas tight performance. Therefore, there are limits as to how fast hydrogen and/or for how long hydrogen can be removed from the compressed tank in a fuel cell system.
FIG. 1 is a cut-away, cross-sectional view of a compressed hydrogen storage tank 10 of the type discussed above. The tank 10 includes an outer structural layer 12 typically made of a graphite composite and an inner liner 14, typically made of a durable plastic, such as a high density polyethylene. The liner 14 provides the gas tight environment for the hydrogen, and the outer layer 12 provides the structural integrity for the compressed hydrogen gas. A metal boss 22, typically stainless steel, is provided between an opening 24 in the outer layer 12 and a neck portion 20 of the liner 14. An adapter 18 is mounted in the neck portion 20 of the liner 14 where a flange 26 of the adapter 18 abuts against an end of the neck portion 20, as shown. The adapter 18 is fitted in the tank 10 and remains in place. A connector 16 is threaded into an outer end of the boss 22 to be positioned against the flange 26. The connector 16 may also extend through the adapter 18 into the liner 14. The connector 16 may contain certain components, such as valves and sensors. The boss 22 is configured to be securely held between the outer layer 12 and the liner 14, to securely hold the adapter 18 to the neck portion 20, and to securely hold the connector 16 within the boss 22. An O-ring 28 provides a seal between the neck portion 22 of the liner 14 and the adapter 18. Additionally, an O-ring 30 provides a seal between the flange 26 and end of the connector 16, as shown. The O-rings 28 and 30 help provide the sealing between the various elements of the connection area; especially at lower pressures. Other tank designs use O-rings at other locations.
It is important that the compressed hydrogen stored in the tank be prevented from leaking or diffusing out of the tank. Because hydrogen is a light and diffusive gas, sealing the leaks is typically difficult, especially around the connection area to the outside of the tank. Thus, it is desirable to reduce the number of seals and the complexity of the connection area. Also, typically the connecting structures in the tank are made of different materials, which also makes the sealing even more difficult.